Lithium secondary batteries have been commercialized and used widely as nonaqueous-electrolyte secondary batteries. Furthermore, lithium secondary batteries are recently attracting attention not only as small batteries for portable electronic devices, but also as batteries for larger-capacity devices such as those in automobiles and for power storage. Thus, the requirements for the batteries, for example in stability, cost, and lifetime, are becoming higher.
The main components of a lithium secondary battery are a positive electrode, a negative electrode, an electrolyte solution, a separator, and an outer casing. The positive electrode constituted by a positive electrode active material, a conductive material, a current collector, and a binder (binding agents).
Generally, the positive electrode active material used is a layered transition metal oxide such as LiCoO2. However, the layered transition metal oxide easily loses its oxygen in the full-charge state, even at a relatively low temperature of around 150° C., and the oxygen removal may possibly lead to thermal runaway reaction of the battery. Thus when a battery containing such a positive electrode active material is used for portable electronic devices, it may result in heat development of the battery and also cause troubles such as firing.
Thus, there is high anticipation for lithium-containing composite oxides, such as lithium iron phosphate (LiFePO4) having an olivine structure, that have a stable structure and do not release oxygen under abnormal conditions, and are less expensive than LiCoO2. Lithium intercalation/deintercalation reaction proceeds in LiFePO4 in so-called a two-phase reaction, and the volume change rate is higher at about 7% between in the lithium-intercalated phase and in the lithium-deintercalated phase. During the two-phase reaction in the lithium-intercalated and deintercalated phases, the plane (bc1) (2 in FIG. 1) defined by the b axis and the c axis of the lithium-intercalated phase of LiFePO4 shown as 1 in FIG. 1 and the plane (bc2) (4 in FIG. 1) defined by the b axis and the c axis of the lithium-deintercalated phase shown as 3 in FIG. 1 forms an interface, and the interface moves by intercalation and deintercalation of lithium. Because the area bc1 in the lithium-intercalated phase is larger by 1.7% than that of bc2, such a device is known to be deformed and have for example cracks along the be plane during repeated charging and discharging, leading to deterioration of the capacity (e.g., Nonpatent Literature 1). The deterioration of capacity is the decrease of capacity over time by repeated charge/discharge cycles.
Various proposals were made to overcome the problem of the deterioration in capacity. For example, Patent Document 1 proposes prevention of the deterioration of capacity by adding Al2O3, which does not contribute to charging and discharging, to the positive electrode. Alternatively, Patent Document 2 proposes prevention of the deterioration of capacity by increasing the dispersibility of the positive electrode active material by adding an inorganic material that does not contribute to charging and discharging of the positive electrode.